1. Field of the invention
The present invention relates to an optical-optical modulator in a broad sense that directly modulates intensity of external light by a light signal. The optical-optical modulator of the present invention includes an optical intensity modulator, an optical amplifier, an optical analog/digital convertor and so on.
2. Description of the Related Art
Optical-optical modulators
The optical-optical modulator utilizes light as a carrier wave and a control signal, i.e. a modulation signal. The modulator is constituted both by an electro-optic device whose optical properties are altered by electric field and by an photovoltaic device which generates voltage by light power. Two representative optical-optical modulators are differentiated in their constitution described below.
A type
The optical-optical modulator of A type disclosed Japanese Patent Publication No. Sho 60-44647 is constituted both by an electro-optic device of directional coupler type and by an anomalous photovoltaic device which generates dozens of voltages made of semiconductor film of CdTe, GaAs, Si and so on fabricated by oblique vapor deposition.
B type
The optical-optical modulator of B type disclosed Applied Physics Letter (Vol.45, No.3, pp 214 to 216, 1984) is constituted both by an electro-optic modulator of Maeh-Zehnder type and by an integrated photovoltaic device of amorphous silicon celIs. The saturation voltage of each cell is around 0.8 V and each cell is arranged in series.
The A type optical-optical modulator has advantages of requiring small amount of intensity of the control light which is incident into the photovoltaic device and enabling to utilize semiconductor laser light around the near-infrared region in which transmission loss optical fiber is a little. However, switching time, or especially the breaking time of an optical control switch, is slow taking several minutes since inner resistivity of the photovoltaic device is high. When CdTe, GaAs, and so on are utilized for the photovoltaic device, its fall time takes several minutes in contrast to its quick rise time taking a few seconds.
On the contrary to the A type modulator, the B type optical-optical modulator is characterized by its quick response time of micro-second. However, two problems exist in the B type optical-optical modulator. First, photovoltaic energy of the Si battery is weak. Since each cell of the Si solar batteries generates just around 0.8 V photovoltage, dozens of the batteries have to be integrated by being connected in series for enough voltage. Consequently, the manufacturing process of the B type modulator becomes complex requiring integration process of the Si batteries. Second, sensitivity of the amorphous Si solar battery is not enough. Since the Si battery is not sensitive to the light of near-infrared region which is 700 nm or more, the B type modulator cannot detect the light with wavelength of 830 nm which is widely used as the light of a laser diode and whose transmission loss is smaller in an optical fiber cable.
Optical-optica A/D convertor
An optical-optical A/D converter is theoretically possible utilizing the optical-bistable device. Since the output light of the optical-bistable device alters in grade depending on increase of optical input intensity, utilizing that characteristic will be able to convert an analogous optical input signal into a digital output signal. However, the threshold value of the optical input intensity for modulation in digit is quite large at the present technological level, so that characteristic is impossible to be obtained within the range of ordinary light intensity. Consequently, the optical-optical A/D converter is not realized yet.